In general, a nickel oxide powder is produced by calcining a nickel salt such as nickel sulfate, nickel nitrate, nickel carbonate, or nickel hydroxide or a nickel metal powder in an oxidizing atmosphere with the use of a rotary furnace such as a rotary kiln, a continuous furnace such as a pusher furnace, or a batch furnace such as a burner furnace. Such a nickel oxide powder is used in a variety of applications as a material for use in electronic parts, electrodes for solid oxide fuel cells, or the like.
For example, when widely used as a material for electronic parts, a nickel oxide powder is mixed with other materials such as iron oxide and zinc oxide and then sintered to produce ferrite parts or the like. When a composite metal oxide is formed by reacting two or more materials by sintering a mixture of the materials as in the case of the above-described production of ferrite parts, its formation reaction is limited by the diffusion reaction of a solid phase, and therefore a powder having a fine particle size is generally suitable as a raw material. It is known that the use of such a raw material increases the probability of contact with other materials and the activity of particles so that the reaction uniformly proceeds even by low-temperature and short-time treatment. Therefore, the use of a raw material powder having a fine particle size is an important factor for improving the efficiency of forming a composite metal oxide.
Solid oxide fuel cells are expected to serve as new power generation systems from the aspect of both environment and energy, and a nickel oxide powder is used as an electrode material for such solid oxide fuel cells. In general, a cell stack of a solid oxide fuel cell has a structure in which single cells each having an air electrode, a solid electrolyte, and a fuel electrode are laminated in this order. As the fuel electrode, one obtained by, for example, mixing nickel or nickel oxide and stabilized zirconia as a solid electrolyte is commonly used. In the fuel electrode, nickel oxide is reduced to nickel metal by a fuel gas such as hydrogen or hydrocarbon so that a three-phase boundary between nickel, solid electrolyte, and gap functions as a reaction field for the fuel gas and oxygen during power generation. Therefore, as in the case of the production of ferrite parts, the use of a raw material powder having a fine particle size is an important factor for improving power generation efficiency.
Meanwhile, the specific surface area of a powder is sometimes used as a measure for determining whether the powder has a fine particle size. Further, it is known that the particle size and specific surface area of a powder have a relationship represented by the following calculation formula 1. The relationship represented by the following calculation formula 1 is derived assuming that particles are perfectly spherical, and therefore there is a certain amount of error between a particle size determined by the calculation formula 1 and an actual particle size. However, as can be seen from the calculation formula 1, the larger the specific surface area is, the smaller the particle size is.particle size=6/(density×specific surface area)  [Calculation Formula 1]
In recent years, there has been a demand for higher-performance ferrite parts, and a nickel oxide powder has come to be used in a wider range of applications such as electronic parts other than ferrite parts, and therefore there has been a demand for a nickel oxide powder having a lower impurity element content. Particularly, among impurity elements, chlorine and sulfur are regarded as elements that are preferably minimized, because there is a case where chlorine and sulfur react with silver used in an electrode so that the electrode is degraded or chlorine and sulfur corrode a sintering furnace.
On the other hand, JP 2002-198213 A (Patent Literature 1) proposes a ferrite material produced from a ferrite powder whose sulfur component content is 300 ppm to 900 ppm in terms of S and chlorine component content is 100 ppm in terms of Cl at the stage of raw material. Patent Literature 1 states that this ferrite material can have a high density even when the ferrite powder is sintered at a low temperature without using any additive, and a ferrite core and a laminated chip component produced from the ferrite material can have excellent moisture resistance and temperature characteristics.
As described above, there is a demand for a nickel oxide powder having lower chlorine and sulfur contents. Further, in the case of a nickel oxide powder for use as a raw material for electronic parts, especially ferrite parts, its sulfur content is required not only to be reduced but also to be strictly controlled to be within a predetermined range. That is, a nickel oxide powder for use as a material for electronic parts is required to have a fine particle size, a lower impurity content, and in addition, a strictly-controlled sulfur content.
As a conventional method for producing such a characteristic nickel oxide powder, a method has been proposed in which nickel sulfate as a raw material is roasted. For example, JP 2001-32002 A (Patent Literature 2) proposes a nickel oxide powder production method in which nickel sulfate as a raw material is subjected to first roasting at a roasting temperature of lower than 950 to 1000° C. and then second roasting at a roasting temperature of 1000 to 1200° C. in an oxidizing atmosphere with the use of a kiln or the like. Patent Literature 2 states that a nickel oxide fine powder having a controlled average particle size and a sulfur content of 50 mass ppm or less can be obtained by this production method.
Further, JP 2004-123488 A (Patent Literature 3) proposes a nickel oxide powder production method in which the step of dehydrating nickel sulfate by calcination at 450 to 600° C. and the step of decomposing nickel sulfate by roasting at 1000 to 1200° C. are clearly separated from each other. Patent Literature 3 states that a nickel oxide powder having a low sulfur content and a small average particle size can be stably produced by this production method.
Further, JP 2004-189530A (Patent Literature 4) proposes a method in which nickel sulfate is roasted at a maximum temperature of 900 to 1250° C. using a horizontal rotary furnace while air is forcibly introduced into the furnace. Patent Literature 4 states that a nickel oxide powder having a low impurity content and a sulfur content of 500 mass ppm or less can be obtained also by this production method.
However, all the methods disclosed in Patent Literatures 2 to 4 have a drawback that the nickel oxide powder has a coarse particle size when the roasting temperature is increased to reduce its sulfur content and the nickel oxide powder has a high sulfur content when the roasting temperature is decreased to make its particles fine. Therefore, it is difficult to control the particle size and the sulfur content to be their optimum values at the same time. Further, these methods have a problem that a large amount of gas containing SOx is produced during heating and therefore expensive treatment equipment for removing SOx is required.
A nickel oxide fine powder can be synthesized also by a method in which an aqueous solution containing a nickel salt such as nickel sulfate or nickel chloride is neutralized with an alkali such as an aqueous sodium hydroxide solution to crystallize nickel hydroxide and then the nickel hydroxide is roasted. In the case of such a method, the amount of an anion component-derived gas produced by roasting nickel hydroxide is small. Therefore, it is considered that exhaust gas does not need to be treated or can be treated by simple equipment, which makes it possible to produce a nickel oxide fine powder at low cost.
For example, JP 2011-042541 A (Patent Literature 5) proposes a method for obtaining a nickel oxide powder having a low sulfur content, a low chlorine content, and a fine particle size, in which an aqueous nickel chloride solution is neutralized with an alkali to obtain nickel hydroxide, the nickel hydroxide is heat-treated at a temperature of 500 to 800° C. to obtain nickel oxide, and the nickel oxide is slurried and then pulverized and washed at the same time with a wet jet mill.
However, in the case of the nickel oxide powder production method disclosed in Patent Literature 5, since nickel chloride is used as a raw material, a reduction in the sulfur content of a nickel oxide powder can be achieved, but it is difficult to control the sulfur content to be within a predetermined range. Further, since nickel oxide is wet-pulverized, there is a fear that agglomeration occurs during drying. Further, a drying step needs to be performed after pulverizing, which is disadvantageous also in terms of cost.
As describe above, it cannot be said that a nickel oxide powder obtained by such a conventional technique is a satisfactory nickel oxide powder having a fine particle size, a low chlorine content, and a controlled sulfur content, and therefore there has been a demand for further improvement.